Adaptation to any dysfunctional and injurious environmental change can cause a living organism to enter into a state of inactivity and suspended animation, where metabolic rates of the individual organs of the body are reduced proportionally to save energy. Winter sleep, or hibernation, with almost a stop in visible activities, is common and is recognized even in animals as big as bears. A similar toning down of metabolic activities occurs during extremely hot weather - 'summer sleep' or aestivation - and is usually seen with amphibians like lizards and frogs. Earthworms, bees, beetles and snails show likewise activity. This is called "Brumation" or "Torpor", as this is colloquially called, and involves physiological changes related especially to body temperature, metabolism, and water balance. The duration of the state of torpor is variable and may last for a few hours, </ a day, a few consecutive days, and a season or more (as seen in some lungfishes and frogs). The organism is reactivated spontaneously once the environmental factors become suitable. The terms "hibernation" and "aestivation" refer to known seasonal variations, while "Brumation" or "torpor" differs. This is almost instantaneous and requires no preparation, usually short-lasting, though the time cannot be predicted, and initiates the sluggishness or cessation of the organism's metabolic activities. In seasonal hibernation or aestivation, preparatory requirements are required for a host of physiological changes related especially to body temperature, metabolism, and water balance of the body that are broken down for the consumption of the minimal energy required for subsistence. Simply speaking energy stores of the body is shored up to withstand a long and grueling unsuitable period where there is no feeding.
It is the ambient surrounding environmental temperature that determines the enzymatic activity within the body and thus O2 consumption and metabolism. Organisms can be of the following types based on the tolerance of temperature –
Stenothermal animals – organisms that can withstand a narrow range of temperatures. These are the types of animals that can live only at some temperatures or in a narrow range of temperatures. The temperature to which such animals adapt varies from one to another species. For instance – Pythons, Penguins, and Crocodiles.
Eurythermal entities – Living beings that can withstand a wide range of temperatures. For instance – dogs, tigers and other cats.
Thermophilic creatures – These can survive at comparatively higher temperatures, typically not less than 20℃. For example – some species of insects, reef corals etc.
Cryophilic organisms – these entities can survive at lower temperatures only. At times it can go as low as 0℃. For instance – salmon, some species of seals, and arctic crustaceans.
Extremophiles – Some of the living entities have a high level of tolerance for extreme temperatures. Such species occupy habitats that have extreme conditions. These are referred to as extremophiles. Example – Thermophilic bacteria can survive in sulfur volcanic hot springs.
Again, the terms Ectothermic and Endothermic are used according to the organism's ability to be affected by external environmental temperature or retain temperature. Ectothermic creatures have a wider range of survival than those retaining them, i.e., the endotherms.
Homeotherms are endothermic and have symptoms beyond a narrow temperature range. Poikilotherms, on the other hand, can change the body temperature consonant with the environmental temperature. Essential enzymatic activity starts on attaining a particular temperature, by sunbathing in some animals, insects, and birds, and the organism becomes active. When adverse conditions are encountered and food becomes scarce, the phenomenon of reduced metabolism with low oxygen consumption becomes essential for survival. It has been observed that mammals gorge on food and store fat necessary for a burn before settling in a cosy and camouflaged hideout during the 'winter sleep' or hibernation.
Cardiovascular surgery changed the surgical perspective. During the initial years, extensive studies were done in the laboratory about the thermal management in bodies. It was a recognized fact that oxygen consumption was low in the cool state. Observation of hibernation in animals and the fact that neurological recovery was complete, encouraged scientific experiments and the surgical time could be increased somewhat. The scourge of death of neural tissue controlling higher function remained till the advent of the heart-lung machine with the provision of continuous neural blood supply. Temperature lowering was practised in conjunction, as by this time, low oxygen consumption and metabolism preserved organs better.
Bigelow started dog experiments in the laboratory and was able to achieve an extension of the survival time by 15-20 minutes when cooled anaesthetized dogs were slowly rewarmed. Bigelow, a Canadian cardiac surgeon, was focused on the preservation of the heart and an increase in the operative time because of his interest. This was in the period 1941 onward, and just after the 2nd war. Heart surgery was in its infancy, developments were slow, closed heart procedures were cornerstones, Harken had returned from war with a vast experience about extraction of bullets and shrapnel embedded in the heart, and open-heart surgery using a heart-lung machine was around the corner. At this time, Bigelow reported his findings on hypothermia with the caution that for acceptable recovery rewarming should be slow and the result at that period was unpredictable. He continued the experiments and was able to convince fellow cardiac surgeons of the lessening of oxygen consumption and metabolic rate in hypothermia. Thereafter the heart-lung machine came, and this was combined with hypothermia for better results.
The research in hypothermia and the relevant physiological effects of thermal variation in the body was carried forward by an American surgeon Henry Swan. He worked mainly with the aestivate African lungfish and NASA funded the laboratory work as they were interested in knowing whether thermal regulation had any bad effect on the astronauts and whether their stay in space stations was prolonged by lowering the temperature. The suspended animation state of some creatures fascinated researchers, and based on the belief that the phenomenon was hormonal, Swan and his associates extracted an anti-metabolic "antabolone" from the brain of the 'lung' fish. Antabolone when injected in laboratory rats, induced the suspended animation state with the attending reduction of temperature by 5° Celsius and the metabolic rate by 35%. Their findings confirmed the presence of a similar substance in other hibernators like the ground squirrel. Later meticulous research, even involving genetic analysis, was done after Swan's active period, but still, a substance-inducing suspended animation in the human race remains elusive.
Hypothermia was used extensively in cardiovascular surgery by this time. A uniform non-crystallizing cold temperature protected the heart. The operative time could be prolonged and unhurried suture placements made the exercise worthwhile. In selected situations and major vascular surgeries, a period of deep hypothermic circulatory arrest was added to facilitate the process and cognitive recovery after the surgery was normal. It has to be understood clearly that optimal metabolic activities of the body occur within a narrow temperature range. The various levels of hypothermia are: --
HT I: Mild Hypothermia, 95-89.6 degrees.
HT II: Moderate Hypothermia, 89.6-82.4 F degrees.
HT III: Severe Hypothermia, 82.4-75.2 F degrees.
HT IV: Apparent Death, 75.2-59 F degrees.
HT V: Death from irreversible hypothermia.
Life is physically not compatible with 92° F and lower, but in surgery, patients are under anaesthetized, and thermoregulation is manipulated according to surgical need. The role of hypothermia here is prolongation of operative time and protection of vital organs with low metabolism and oxygen uptake. In cardiac surgery, the goals are:-
1. Extension of the operative time.
2. Myocardial protection
3. Cerebral and other vital organ protection, and
4. Lessening the chance of infection.
5. Reduction in bleeding in the operative field.
Deep hypothermic circulatory arrest (DHCA) is needed in major vascular surgery and dissection and control of bleeding in certain difficult situations. These are hopeless conditions with dire consequences and surgical palliative repair is the only hope and DHCA is done knowing fully that cognitive recovery may not be absolutely as before. Satisfactory neural recovery occurs when the arrest time does not exceed 40 minutes.
Thus, hypothermia for surgical purposes is classified as:
1. Mild - with the temperature range of 37° to 35° C.
2. Moderate - ............................................ 35°to 28° C, and
3. Severe - .................................................. 28° C and less.
Deep hypothermic circulatory arrest (DHCA) is practised in the narrow range of 20° to 16° Celsius (C). The recent trend has been an antegrade or retrograde cerebral flow in addition to a period of DHCA for better cognitive recovery.
Radiation, conduction, convection, and evaporation are the main methods of heat regulation of the body. 60% of heat loss occurs by radiation. Conduction and convection (15%) are also involved and evaporation from tissue on exposure accounts for 22% of temperature loss. Anesthesia robs the patient of the regulatory reflex mechanism and exposure causes the temperature drift to subnormal levels. Surgical access incisions and exposure of tissue also lead to temperature loss, but this is recoverable. Maintenance of a core temperature of about 37° C is needed for meaningful recovery and protective measures should always be in place for preventing prolonged hypothermia. An intraoperative warm continuous water-circulating mattress is always on the operating table. When a cardiac operation is done under cardio-pulmonary bypass, a gradient not exceeding 10° C between the core temperature and the perfusate is strictly maintained. After closure, warm blankets inflated with hot air are placed over the patient remembering to keep the gradient (skin & blown air) within a range thereby preventing burns. The goal is to approximate core and surface temperature as the patient recovers from the effects of anaesthetic medicines. The organisms' defence thermoregulation mechanism sets in by this time and normalizes metabolic responses.
Most open-heart procedures are done with mild to moderate hypothermia. Seldom the surgeon has to go lower than 28° C. (DHCA) is reserved for special situations only. One should consider this important that a diastolic arrest of the heart is required for repair and a cardioplegic solution is necessary. Cardioplegia, antegrade/retrograde or combined, lowers the temperature of the heart as well and thus is in synergy with the hypothermic methods employed. Blood cardioplegia became popular later with the belief of the addition of haemoglobin and plasma protein in the right concentration helps cellular metabolism during arrest. There was a period when debates and discussions about the superiority of warm-blood cardioplegia were popular. The temperature of cold cardioplegic solutions hovers around 4° C while that of warm blood cardioplegia is about 35° C. Hence this is on the lower side and does little to contribute to the hypothermic efforts. Hypothermic methods are continuous and thus prevent sudden rewarming of the heart. The outcome may not be favourable, and recovery with lowered ejection fraction and the risk of fatal arrhythmogenicity is always there.
Cardiovascular surgery is a massive effort and involves a team of personnel including the anaesthetists, perfusionists, students, and nursing personnel. In addition, there may be observers can be there and they increase the number of persons inside the O.T. Thus, the temperature of the operation theatre is multi-factorial --
1. Overhead light and other illuminations as required.
2. The number of persons in the operation theatre increases the surrounding temperature by radiation, reflection between each other, clothing, etc.
3. Heart generated by the monitoring and other radio-frequency generators, motor of suction machines and other instruments.
Independently these may seem innocuous, but when considered together a significant change is possible making maintenance of the core temperature difficult. Postoperative differences between core and cutaneous temperatures, when monitored, indicate mainly the temperature drift due to exposure and the circulating volume status. Approximation of the temperature recordings and careful titration of the volume status led to hemodynamic stability and early recovery. Central air-conditioning and laminar airflow with the proper usage of high energy particulate arresting (HEPA) filters limit infection along with the maintenance of an ambient environment of the operation theatre.
The efforts so far have been to create artificially a temperature at which the organism will survive with the lowest possible metabolic activity. This allows valuable additional time, and an unhurried repair is possible with a low probability of infection. Maintaining a steady and sustainable core temperature is desired though even the core temperature will show a downward trend with time.
There are many ways to measure the core temperature. The rectal temperature gives the closest approximation. However, soiling and morbid infection are a probability. Efforts have been there to mount the tip of a Swan-Ganz catheter with a thermistor to record pulmonary arterial temperature, but the process is cumbersome and requires a trained operator. Similar issues arose when mid-esophageal temperatures were suggested as the temperature nearest to the heart operated could be recorded. The axillary and the tympanic membrane temperatures are preferred by many surgeons as temperature recording can be done adjacent to arterial blood flow. Mal-positioning and the risk of tympanic membrane perforation are deterrents. The pharyngeal temperature is a near approximation of the core and easy to place after anaesthetic induction and endotracheal intubation. This is now the preferred method. The skin temperature is also recorded and an approximation of the peripheral and core temperatures in the immediate post-operative period suggests hemodynamic stability in otherwise normal situations.
Blood gas analysis and consequential pH management of the circulating blood is a debatable subject. The majority interprets the results as they come, i.e, the results are uncorrected for a temperature change. This is satisfactory for usual cardiac cases even when surgery is done under mild and moderate hypothermia. This is known as the alpha-stat strategy where it is presumed that at a pCO2 of 40 mm of Hg pH of blood is 7.4. Some people opine that temperature-related corrections are essential for better management. Continuous infusion of 100% CO2 in small amounts with an additional graduated flow meter on the blender in the gas sweep line containing a mix of air and oxygen, is started. CO2 is more soluble in low temperatures and interferes with solubility of O2 and other gases. A varying amount of 3-5% of CO2 is enough to maintain a fixed pCO2 of 40 mm of Hg and the pH of circulating blood at 7.4. The early '60s saw a rise in the adoption of this pH-stat method of blood gas analysis and cardiopulmonary bypass as it was shown in scientific journals that there was some vasodilatation during hypothermia and the flow of blood to the brain increased. Clinically the current trend is mixed with scientific data showing that with these advantages, there is also an increased incidence of microembolization, consequential coagulation defects, and cerebrovascular insults. It appears that it is more physiological to use the alpha-stat strategy whenever mild or moderate hypothermic CPB is used to maintain intracellular electro-chemical neutrality and to adopt the pH-stat strategy whenever deep hypothermic circulatory arrest is induced, to optimize brain protection. Better cognitive recovery has made cardio-thoracic and vascular surgeons adopt the pH-stat strategy in pediatric congenital surgery and a mix of alpha-stat at induction with pH-stat for prolonged maintenance in other operations. pH-stat strategy has been universally adopted for a procedure electively under deep hypothermic circulatory arrest.
Temperature lowering diminishes tissue O2 demand and the adoption of strategies for maintaining the cellular electro-chemical balance at near normal levels helps in lowering cerebrovascular and myocardial accidents with better cognitive recovery. Sustaining life in a state of suspended animation could be the future goal.
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